It may be quite a while before I get round to trying to make film holders, so don't hold your breath
But in principle I don't see it as being very difficult. What occurred to me as I held one and looked at what it does and how it does it is that it only looks complicated.
Once I visualised it as a sequence of layers of material stacked up (rather than a complex monoblock) it became much easier: consider each required element separately, and it becomes much more apparent that one can be made by glueing together the parts in layers.
So for instance, the groove in which the darkslide runs looks impossible - how do I make a groove like that? But considered as three strips of material stacked up with the centre one a little less wide than the ones above and below, it's suddenly quite straightforward.
Working out how to build the loading end seems the most complicated part, but only if one only tries to copy the way that it is implemented in a standard film holder. One has to make it light-tight and secure, so we can let form follow function. Having said that, I haven't given that bit much thought yet.
Maybe if I have a wet afternoon to fill, I might try prototyping one in foamcore. It'd be about 4 inches thick, but it would be a proof of concept.
I've no doubt others have had similar thoughts and probably if I looked hard enough I could find a few similar designs on the interwebs, but so far I only found ones built by damn clever home woodworkers with a garage full of tools. Whereas I have only a knife, a mat and steel rule